AASHTO SEISMIC T-3 Columbus, Ohio Meeting

2nd Edition Guide Specification

Applicability to Towers and Arches

June 23, 2014

David Goodyear & Hans Lund, TY Lin International

Earthquake Resisting Systems

Ductile substructure w/elastic superstructure

Elastic substructure w/ductile superstructure

Steel superstructure only

Ductility only at pier x-frames

Elastic super and substructure only with fuse in between

(base isolation)

All referenced to beam bridge systems

Section 3.3

Question

How should Designers apply the Guide Spec when

other lateral loads control?

Is the strong beam logic of the Guide Spec applicable

to all bridge types (other than beam bridges)?

Is CP design appropriate for towers with deep, flexible

foundations or long span arches?

Historical Provisions

Past applications for towers and arches (LRFD, LFD) could be consistently applied within Code framework

Concepts for towers – weak beam systems (eg, shear links or portal frames)

Pseudo-elastic system based on elastic (unreduced) demands

ATC 49 Section 4.10 – specific criteria

VS 2nd Edition 4.11.1 – open commentary

History for Ductility Based Design

Peak forces reduced for inelasticity/ductility

Inelastic displacement demand similar to elastic displacement

Elastic (unreduced) forces predicted by RSA

Displacement predicted by RSA

Fe

dy dp

Fy

Why Pseudo-Elastic Seismic Design

Where inelasticity is not desirable

Limited or no damage criteria

Determinate systems where inelasticity may compromise

structural stability

Where elastic seismic design forces do not control

design

Where seismic over-strength (CP) exceeds reasonable

design earthquake demand level; design impractical

In Practice

Longer period structures:

Lower seismic demand

Higher wind demand

ASCE 7 Wind

Longer period yields higher wind

gust forces (opposite of EQ)

Not all Designs controlled by EQ Any bridge with tall piers in a moderate seismic environ may be

controlled by wind

A bridge with heavy marine traffic may be controlled by vessel impact

Single pylons on deep foundations with flexible girders should not have hinges

Application of capacity protection

concepts based on Guide Spec could

lead to irrational over-design

1.4Fw

Fw

Fe

FR

δp δy δover

Current Limitations

Presumes all substructures governed by seismic

Prescriptive systems limit performance based design

approach

Weak column requirements limit rational application to

conventional beam bridges – not advised for towers or

arches

Result: We do not have a codified approach for many

major bridge design sites. We use ad-hoc project

criteria.

Revision to Guide Spec

In Section 3.3

Pseudo-elastic system for

unreduced seismic

demands (historical

system with NL analysis)

Ductility detailing per CP

design rules

Add foundation design

provisions

Exception for structure

types

Clarify (or stipulate) TH

methods in lieu of 4.11.1,

with 1.2 factor on demand

displ for no liquefaction,

and 1.2 on motions if

liquefiable

Current 4.11.1

Provision and Commentary Proposed Provisions – end of 4.11.1:

Inelastic capacity protected design of columns (hereinafter representing towers, arch

ribs and related substructure elements) may be omitted for the following cases:

•Where non-seismic lateral load demands require essentially elastic design in

excess of the section requirements for an inelastic design for seismic loads;

•Where inelastic design of vertical support elements would result in geometric

instability of the structural system either during or after a seismic event;

•Where the flexibility of deep foundations will not develop the plastic moment

capacity of columns or towers designed for non-seismic loads that are in excess of

the elastic seismic demand levels as described below.

In cases where ductility based column design is not provided per this Article, the

methods of Section 5.4.4 shall be used to determine seismic response for design.

Moment-curvature definitions shall be developed for all elements, including

substructure elements subject to non-linear behavior (geometric or material).

Capacity protected elements shall be designed for essentially elastic behavior based

on 1.2 times the demand displacements or 1.2 times the ground motions derived

from the non-linear time history analyses for the event suite defined in Section 5.4.4.

Nonlinear Foundation Analysis

Depth-varying

ground motions

and soil springs

Pier-specific:

based on soil

conditions and

site response

Structural Model Basis Moment-Curvature Elements

Towers modeled with moment-

curvature elements throughout (excl

anchor box)

Piles modeled with moment-

curvature elements (non-composite

at cap interface)

Strain Distribution Through Strain

Hardening (versus explicit hinge L)

Basis of Design

Time-History Design Basis Non-linear modeling of both structure and soil

Lateral Spreading directly applied to structure through motions (permanent drift)

Direct measure of demands for expansion joints, dampers, etc.

Monitoring of NL strain demands in foundation elements.

Implementation of foundation protection

Plastic hinging might not be appropriate as discussed when not governed by EQ

1.2 times displacement demand as standard; may not appropriately capture demands at depth

(liquefaction forces); OR

1.2 times motions; do not compound margins applied to ground motions (liquefiable sites)

Discussion